This invention relates to the manufacture of seamless metal tubing and concerns a new continuous hot rolling method and the apparatus for implementing such method.
The manufacture of seamless metal tubing by hot rolling in a continuous mill generally includes the following steps: hot piercing of a solid round billet of a given length having been previously heated to a given temperature, in order to get a tubular heavy-wall blank or shell having already undergone a first elongation, and then hot rolling the obtained shell in a continuous mill which delivers a tube of controlled diameter and wall thickness. Later on, the tube is generally subjected to a number of additional hot or cold operations performed in order to meet the required specifications.
Depending on the size of the incoming billet and the size of the tubes to be produced, an additional intermediate elongation operation can be performed between piercing and rolling, intending to condition the shell delivered by the piercing mill in order to obtain a new shell making it possible to use the continuous mill under good conditions.
Piercing is generally performed in a rotary type piercing mill. The principle of this mill is to push the solid round billet by transverse rolling which develops axial force to drive the billet over a piercing plug which is axially held in position by the piercing bar over which the shell leaving the mill moves.
Rolling is performed in a continuous mill composed of a number of successive roll stands aligned in roll pass centerline, the planes of symmetry perpendicular to the roll axes of alternate stands being disposed at 90.degree..
The number of roll stands used in a mill is variable. It depends on the elongation ratio to be achieved during the rolling. For an elongation of 4.5 to 1 generally 8 stands are employed, the elongation being the ratio of the length of the rolled tube to the length of the incoming tubular shell.
Each stand is equipped with two driven rolls having grooves of symmetrical profile with a more or less pronounced side relief so as to permit metal flow and deformation to take place under good conditions.
Several techniques or methods are available for continuous rolling of seamless tubing in this type of mill: rolling over a full-floating mandrel and rolling over a retained mandrel.
These methods implement a mandrel over which the tubular shell is being rolled as it passes through the successive stands, the main difference lying in the way the mandrel is moved during the rolling operation.
In the method of the full-floating mandrel, the tubular shell with its long mandrel inside is inserted into the inlet roll stand of the mill and the mandrel takes an average speed which is the resultant of the speeds of the tube being rolled at every roll stand.
The rolled tube partially covering the mandrel is collected at the exit of the mill and mandrel stripping is then performed.
Thus, in this method, the mandrel is not connected to any mechanical or other speed control device during the rolling operation.
The limits and draw-backs of this process are well known. There can be mentioned: limitation of the length of the rolled tube unsteady working conditions as the product enters and leaves the mill and, thus, tube size variations for the corresponding cross section, relatively long mandrel length, and mandrel stripping difficulties leading to rejections due to stripping incidents mainly for thin-wall tubing.
In the method of the retained mandrel, the pierced shell with its long mandrel inside is inserted into the inlet roll stand of the mill, but then the mandrel is retained and moved during rolling over a distance corresponding to twice the roll stand center distance which is generally considered at the last stands of the mill. In this method the mandrel is thus connected to a mechanical or other speed control device which holds the mandrel and forces it to move at a speed less than its natural speed rate.
Thus, one always manages to have the tube rolled over the mandrel as the tube and the mandrel proceed through the mill, but as the mandrel only moves over a short distance, it is moved at a very slow speed that is to say at a speed rate considerably less than the linear shell entry speed into the mill and generally much less than 50% of that speed.
This results in severe rolling conditions, and special provisions must be made when manufacturing and using the mandrel in this method.
This basic difficulty resulting from the difference of the speeds of the mandrel and the shell during the rolling is well-known and has given rise to many publications dealing with the design of the mandrels, their lubrication, their internal cooling, and their surface conditioning.
Among these documents, there can be quoted the French Pat. Nos. 1 224 862 and 1 458 826, the U.S. Pat. No. 3,394,568, and the article of M. Dvorak et al. in BTF-Gennaio-Febbraio 1980, pages 4 and 5.
In the retained mandrel method, the mandrel is much shorter than the one of the first mentioned "full-floating" method. The rolled tube leaves the mill at its exit end, and the mandrel is generally retracted backwards after use.
In spite of the advantages as compared to the full-floating mandrel method, the retained mandrel method, however, also has its limitations and draw-backs. The high relative shell/mandrel speed during the rolling operation results in heating and rapid wear of the mandrel and entails very high operating costs because of the mandrels which burden the process as a whole.
Such a method is described in the French Pat. No. 1 322 304.
In another method which is described in the U.S. Pat. No. 3,857,267 attempts were made to eliminate the draw-backs inherent in the full-floating and the retained mandrel processes by imposing to the mandrel a constant speed all through the rolling operation, this speed being calculated so that the available length is always at least equal to the length strictly required for the considered rolling operation, thus the mandrel can be released through the mill at the end of the rolling operation. This results in an increase in productivity as compared to the retained mandrel process in which the mandrel is retracted backwards after use.
In the method implementing a mandrel moving at controlled speed, various operation conditions can be used for the actual rolling operation.
More particularly it has been proposed to move the mandrel at speeds being variable according to the position of the tubular shell under rolling in order to ensure a product quality as uniform as possible and to solve the above mentioned problems.
In spite of all the precautions that can be taken for operating the retained mandrel process or its alternates, the question of the life of the mandrels in service remains an important problem.